1. Field of the Invention
The present invention relates to an injection assembly for injection moulding machines for plastics material.
2. Description of Related Art
According to the prior art, in the injection moulding procedure, injection moulding machines or devices are used wherein resins, consisting of plastics material, are heated to be brought to a molten state in a heating cylinder and are injected at high pressure into a cavity of a mould so as to fill same. The resin in the molten state is cooled inside the mould so as to harden and form the moulded product. At this point the mould is opened and the moulded is extracted and can be sent to storage or subsequent processing stages.
The injection moulding device comprises a moulding assembly and an injection assembly. The first assembly is generally formed of two moulds halves mounted respectively on a fixed plate and a movable plate, so that the mould can be opened and closed by causing the movable plate to advance and retract.
FIG. 1 shows schematically an injection assembly according to the prior art, denoted as a whole with reference numeral 100. The injection assembly 100 comprises a heating cylinder 2 to bring the resins fed through a hopper 3 to a molten state, and an injection nozzle 4 to inject the molten material into the mould cavity. Inside the heating cylinder 2 is a plastication screw 5 which makes a rotatory movement around its own axis for plastication of the resins, a translational forward movement for injection of the molten material and a translational rearward movement which allows the resins to be introduced from the hopper 3.
The plastication screw 5 is driven by means of a system of electric motors. For the translational movement of the plastication screw, a screw-nut and screw system is provided wherein a screw 6, preferably a ball screw, engages in a screw nut 7. The screw nut 7 is driven by a motor 8 by means of a reduction unit 9 fixed to the frame 10 of the machine.
Rotatory movement of the screw 6 around its own axis is prevented by an anti-rotation device. Said anti-rotation device comprises a rod 11 fixed to the screw 6 and a stop block 12 fixed to the frame 10 of the machine. In this manner the rod 11, abutting against the stop block 12, prevents rotation of the screw 6, allowing only translation. Accordingly, operation of the electric motor 8 in one direction or in the opposite direction allows forward or backward movement of the screw 6, which draws with it the plastication screw 5.
As far as the movement of rotation of the plastication screw 5 is concerned, this is obtained by means of a splined shaft 13 connected by means of a joint 20 to the plastication screw 5. The splined shaft 13 engages in a bush 14 set in rotation by an electric motor 15 by means of a reduction unit 16 fixed to the frame 10 of the machine. The splined shaft 13 is uncoupled from the screw 6 by means of a ball bearing assembly 17, so that the rotatory movement of the plastication screw 5 can be independent of the translational movement imposed by the screw 6.
The electric motors 8 and 15 are provided respectively with encoder type sensors 18 and 19 to carry out the speed adjustments required in the various phases of the work cycle.
The above described injection assembly, according to the prior art, has a drawback due to the fact that the stage of translation of the plastication screw takes place under very high axial loads. Therefore the driving mechanism based only on the ball screw 6 works in very demanding conditions if it is applied to large-sized moulding machines. The problems concern in particular the state of stress of the ball screw 6 determined by the high and variable loads which lead to wear on the screw and screw nut system and compromise the strength of the entire construction.
Said problem can be overcome only in part by replacing the ball screw with a planetary roller screw.
The object of the invention is to eliminate said drawbacks by providing an injection assembly for injection moulding machines for plastics material suitable to be applied to large-sized moulding machines so as to allow high performances and high production capacity.
Another object of the present invention is to provide such an injection assembly for plastics material that is practical, economical, versatile and easy to make.
These objects are achieved in accordance with the invention with the characteristics listed in appended independent claim 1.
Advantageous embodiments are apparent from the dependent claims.
The injection assembly for large-sized presses according to the invention is formed by a fixed base and a mobile structure translating on linear guides provided on the base. The plastication screw and the drive systems of the screw and of the movable structure are connected to the movable structure, whereas the plastication cylinder and the hopper are integral with the fixed part.
In a preferred embodiment of the invention the plastication assembly provides for use of four ball screws rigidly connected to the base, with the relative lead nuts positioned on the movable structure and free to rotate
The movement of pure translation for the injection stage is obtained by simultaneously setting in rotation the lead screws which pull the entire moveable structure and thus also the plastication screw.
To obtain synchronous, concordant movement of the lead nuts, a crown gear that engages simultaneously with four gear wheels directly connected to said lead nuts is used.
The injection assembly according to the invention has various advantages with respect to systems of the prior art.
The injection assembly according to the invention in fact has an extremely simple actuating mechanism, based principally on the use of a plurality of ball screws which allow the high axial load during the injection phase to be shared out. This arrangement offers high guarantees on the strength of the structure, the precision of movement of the plastication screw and the reproducibility of the operations of the work cycle.